The present invention relates generally to a gear pump or a gear motor for a fluid. For the sake of brevity in the specification, the expression rotary gear machine for fluid actuation will be used broadly to cover both a rotary gear pump and a rotary gear motor; in the case of the gear pump, the pump provides for actuation of the fluid, that is to say displacement thereof, while in the case of the motor, the motor is actuated by the fluid.
An example of such a rotary gear machine for fluid actuation may be found in specification FR-A-No. 1 442 211, and comprises a housing having a high pressure port and a low pressure port, together with first and second pressure or thrust plates disposed at respective sides of a pair of meshing gears. The gears are carried on respective shaft portions of which at least one extends outwardly to provide a drive input or output shaft. One side of one of the thrust plates adjoins an intermediate plate while with its other side it bears against the side surfaces of the meshing gears. The intermediate plate is of a generally 8-shaped configuration, co-operating with sealing means to define a plurality of pressure areas for pressing the thrust plate against the side surfaces of the gears in a specifically selected fashion for enhanced operational efficiency of the unit.
In that arrangement, the thrust plates are at the same time in the form of mounting carriers and are of a configuration corresponding to that of a `8`, although with the `necked` configuration thereof being somewhat attenuated in order not to be excessively narrow in the central portion of the 8-shaped configuration. The contour of the intermediate plate corresponds to that of the thrust plates which form the carriers. Provided in a lateral housing portion is a groove for accommodating a seal which is also of a generally 8-shaped configuration, the peripheral contour thereof being generally smaller than that of the thrust plate, although the seal has outwardly extending projections forming ears thereon, which extend to a position beyond the peripheral configuration of the intermediate plate in order to delimit the various pressure areas thereon. More specifically, the pressure areas comprise a central high-pressure area which extends not only axially laterally from the location of external engagement as between the teeth of the gears, but also in an annular configuration around the mounting shaft portions so that generally that pressure area is similar to the configuration of a pair of spectacles. Other pressure areas are arranged outside the outside contour of the seal, insofar as the intermediate plate projects therebeyond. The high pressure areas in the construction further include a high-pressure area which extends in an arc parallel to the tips of the teeth of one gear, part of the way around that gear, and another similar high-pressure area which also extends in an arc parallel to the tips of the teeth of the other gear, part of the way around that other gear. Those two high-pressure areas are combined together as effectively a single area. The actuation surface of the intermediate plate is fairly large and that means that the pressure areas are similarly of substantial extent. That may be a factor of some disadvantage when using high system pressures. Moreover, the system pressure passes through gaps on the rear side of the intermediate plate, such gaps being formed as a result of flexing of the components of the mechanism, under load due to the high pressure. The pressing action of the mechanism therefore is operative only when the difference between the pressures at the inlet and the outlet of the unit exceeds a given level so that due to deformation and displacement of components, the high pressure can reach the locations at which it is required to act.
In another rotary gear machine for fluid actuation, as disclosed in German patent specification No. 1 653 866, the mechanism has a pressure or thrust plate which in the region of its intake and its outlet has a respective peripherally extending seal at the rear side thereof, thereby forming a high-pressure area for urging the thrust plate towards the meshing gears. As pressurised fluid is also to be found in the region between the pressure or thrust plate and the gears, the effect of that pressure area which is disposed in the region of the pressure connection port is slight. In order to provide for an adequate level of contact pressure in that arrangement, the system pressure, which is at a high pressure level, is taken from a gap between the two meshing gears and passed to the rear of the thrust plate by way of communicating passages in the form of grooves. That arrangement therefore makes use of the fact that the gears only have a gap between them at the high-pressure side whereas at the low-pressure side they block off any possible flow of fluid. A disadvantage in that configuration is the fact that the differential pressure as between the inlet and the outlet must exceed a certain level so that the above-indicated phenomenon of a gap between the meshing gears occurs. For example, when operating the machine as a gear motor, it may happen that an increased pressure occurs in the return so that the difference relative to the pressure in the feed is low. In such a case the thrust plates are not adequately pressed against the gears and the result is an extremely high level of external leakage oil.